Switched PluginRegistry to be singleton, because Reflections is not thread safe in jar archives scan

Author: andsel

logstash-core/src/main/java/org/logstash/plugins/discovery/PluginRegistry.java

@@ -37,21 +37,41 @@
 import java.util.Set;
 
 /**
- * Registry for built-in Java plugins (not installed via logstash-plugin)
- */
+ * Registry for built-in Java plugins (not installed via logstash-plugin).
+ * This is singleton ofr two reasons:
+ * <ul>
+ *  <li>it's a registry so no need for multiple instances</li>
+ *  <li>the Reflections library used need to run in single thread during discovery phase</li>
+ * </ul>
+ * */
 public final class PluginRegistry {
 
     private final Map<String, Class<Input>> inputs = new HashMap<>();
     private final Map<String, Class<Filter>> filters = new HashMap<>();
     private final Map<String, Class<Output>> outputs = new HashMap<>();
     private final Map<String, Class<Codec>> codecs = new HashMap<>();
+    private static final Object lock = new Object();
+    private static PluginRegistry instance;
 
-    public PluginRegistry() {
+    private PluginRegistry() {
         discoverPlugins();
     }
+
+    public static PluginRegistry getInstance() {
+        if (instance == null) {
+            synchronized (lock) {
+                if (instance == null) {
+                    instance = new PluginRegistry();
+                }
+            }
+        }
+        return instance;
+    }
 
     @SuppressWarnings("unchecked")
     private void discoverPlugins() {
+        // the constructor of Reflection must be called only by one thread, else there is a
+        // risk that the first thread that completes close the Zip files for the others.
         Reflections reflections = new Reflections("org.logstash.plugins");
         Set<Class<?>> annotated = reflections.getTypesAnnotatedWith(LogstashPlugin.class);
         for (final Class<?> cls : annotated) {

logstash-core/src/main/java/org/logstash/plugins/factory/PluginFactoryExt.java

@@ -82,7 +82,7 @@ public static IRubyObject filterDelegator(final ThreadContext context,
     }
 
     public PluginFactoryExt(final Ruby runtime, final RubyClass metaClass) {
-        this(runtime, metaClass, new PluginLookup(new PluginRegistry()));
+        this(runtime, metaClass, new PluginLookup(PluginRegistry.getInstance()));
     }
 
     PluginFactoryExt(final Ruby runtime, final RubyClass metaClass, PluginResolver pluginResolver) {